import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
import java.util.Scanner;

/**
 * 电子秤
 */
public class ElectronicScale {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double[][] nums = new double[30][30];
        int tmp = 0;
        for (int i = 0; i < 29; i++) {
            for (int j = 0; j <= i; j++) {
                nums[i][j] = scanner.nextInt();
                if (j == i) {
                    tmp += nums[i][j];
                }
            }
        }
        for (int i = 1; i < 30; i++) {
            for (int j = 0; j <= i; j++) {
                if (j == 0) {
                    nums[i][j] += nums[i - 1][j] / 2;
                } else if (j == i) {
                    nums[i][j] += nums[i - 1][j - 1] / 2;
                } else {
                    nums[i][j] += nums[i - 1][j] / 2 + nums[i - 1][j - 1] / 2;
                }
            }
        }
        double[] res = nums[29];
        Arrays.sort(res);
        System.out.println(2086458231 / res[0] * res[29]);
    }
}
